RF linear amplifiers utilize devices that exhibit non-linear characteristics at higher power levels whereby signal distortion is introduced. If more than one signal is applied to a linear amplifier, its non-linear characteristics cause an unwanted multiplicative interaction of the signals being amplified and the amplifier output contains intermodulation products. These intermodulation products cause interference and crosstalk over the amplifier frequency operating range which interference may exceed established transmission standards.
As is well known, intermodulation distortion can be reduced by negative feedback of the distortion components, predistortion of the signal to be amplified to cancel the amplifier generated distortion or by separating the distortion component of the amplifier output and feeding forward the distortion component to cancel the distortion in the amplifier output signal. Of these techniques, the forward feed approach provides the most improvement. Forward feed, however, is the most difficult to apply since it requires modifying the separated distortion component in amplitude and phase to match the gain and phase shift of the amplifier on a continuous basis.
U.S. Pat. No. 3,886,470 issued to O'Neil et al May 27, 1975 discloses a forward feed amplifier system in which an amplified signal from a main amplifier is compared with a time delayed unamplified signal to isolated the noise and distortion components produced by the main amplifier. The isolated distortion and noise components are then amplified in an auxiliary amplifier and combined with the signal amplified in the main amplifier to cancel the error caused by main amplifier distortion. In such a system, it is necessary to adjust the delay and amplitude of the unamplified signal to obtain complete isolation of the distortion and noise components from the main amplifier and to control the phase and gain of the auxiliary amplifier to achieve adequate error cancellation. Such adjustments are difficult to make automatically.
U.S. Pat. No. 4,580,105 issued to Robert E. Myer Apr. 1, 1986 discloses an arrangement for automatic reduction of intermodulation products in high power linear amplifiers wherein a distortion simulating pilot at a preset frequency is injected at the input of an amplifier which uses feed forward distortion correction. The magnitude of the pilot signal in the amplifier output is used to control a decreasing step size controller to adjust the gain and phase of the feed forward distortion signal. In this way, both the pilot signal and the distortion introduced by the amplifier are removed. While the injected pilot permits a high degree of automatic control, it occupies a portion of the amplifier band that could otherwise be employed to carry an information bearing signal and reduces the frequency usage efficiency. It is an object of the invention to provide an improved feed forward amplifier arrangement with automatic reduction of distortion without affecting frequency usage efficiency.